Process of operating fuel cell including alkali-metal and transition-metal containing electrolyte



United States Patent PROCESS OF OPERATING FUEL CELL WCLUD- IN G ALKALI-METAL AND TRANSITION-METAL CONTAINING ELECTROLYTE Robert J. Allen, Saugus, Robert Lee Novack, Hanover, and Henry George Petrow, Cambridge, Mass, assignors to Prototech Company, division of Bolt, Beranek, and Newman, Inc., Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed Aug. 9, 1967, Ser. No. 659,296

Int. Cl. H01m 27/00 U.S. Cl. 136-86 6 Claims ABSTRACT OF THE DISCLOSURE This disclosure involves a method of increasing oxygen utilization and simultaneously increasing power density in electrochemical cells and the like through the introduction of certain transition metals capable of catalyzing peroxide and superoxide formation for utilization in molten electrolytic media while agitating the cathode region of the cells.

The present invention relates to methods of increasing oxygen utilization While simultaneously increasing the power density of electrochemical cells, being more particularly, though not exclusively, directed to such improved operation in fuel cells and the like employing high-temperature molten electrolytic media in which continual peroxide and superoxide formation is attained through agitation in the cathode region of the cell.

In the co-pending application of Robert Novack, David Moulton and Walter Juda, Ser. No. 539,768, filed Apr. 4, 1966, for Fuel Cell Process and Apparatus, a preferred cell of the above-described characteristic is disclosed comprising preferably an alkaline hydroxide, molten auhydrous electrolytic medium provided with a palladiumcontaining hydrogen-permeable, but otherwise impervious anode, a cathode such as non-porous nickel which may, indeed, comprise the cell housing, and an oxygen inlet that enables agitation of the medium in the region of the cathode to produce peroxide and superoxide formation in such region, with an electrolytic-medium-ion-permeable barrier interposed between the anode and cathode of suificient dimensions and sufiiciently restricted porosity to minimize the transfer of agitation from the cathode region of the cell to the anode region. Such cells are preferably operated above about 350 C. in the region of 400-600 C. and oxygen or air (hereinafter generically referred to as oxygen) is bubbled into the cathode region, usually through a nickel or other tube disposed within the electrolytic medium at such region.

In such cells, as well as other similar electrolytic cells, many parameters have been varied to try simultaneously to increase both the oxygen utilization and the power density of the cell, including increasing temperature, varying hydroxide composition and other techniques. In accordance with a discovery underlying the present invention, certain oxidizable transition metal additives have been found admirably to accomplish these ends without the deleterious effects that have heretofore been obtained as accompanying the use of at least one of such additives.

An object of the invention, accordingly, is to provide a new and improved method of increasing oxygen utilization and simultaneously increasing the power density of an electrochemical cell through the use of certain additives and certain critical quantities of same. In summary, this end is attained while agitating the cathode region of the cell through the introduction of oxidizable metals selected from the group of transition metals capable of 3,471,332 Patented Oct. 7, 1969 "ice catalyzing peroxide and superoxide formation or utilization in the electrolytic medium.

Other and further objects are delineated in the appended claims.

Returning to the type of cells described in the co-pending application as illustrative of the application of the present invention (though not restricted thereto), it has been found that the introduction into the electrolytic medium, during the agitating of the region near the cathode, of certain transition metals and in appropriate limited quantity, has resulted in the catalyzing of either or both of (1) peroxide or superoxide formation in the electrolytic medium that serves as the oxidant of the cell, or (2) utilization thereof markedly increasing oxygen utilization in such region and simultaneously increasing the power density of the cell.

Among the oxidizable transition metals that have been so determined to effect this result are manganese, iron and cobalt, the last-mentioned being the subject of copending companion application Ser. No. 659,297, filed Aug. 8, 1967, for Method of and Apparatus for Catalytic Improvement in the Operation of Electrochemical Cells and the Like; these materials being preferably introduced in the form of oxides or nitrates.

No predictable or a priori way is known or available for determining in advance which oxides can successfully increase air utilization in the agitated cathode chamber with simultaneous increase in power density. Oxides of other elements that, like Mn, Fe and Co, are also members of the transition metal family, indeed (such as chromium and vanadium, for example) have been found decidedly not to produce these results.

Prior experience with manganese, for example, in fused hydroxide media, moreover, has shown poor hydrogen utilization; such that it was surprising that when used in the barriered agitated-cathode region cell of the present invention, improved long-term operation has been consistently obtained Without seriously impairing hydrogen utilization.

As an example, in a cell of the above-described characteristics having a palladium anode layer of area 57 cm. a nickel cathode about five times such area, and a percent KOH-lO percent NaOH substantially anhydrous electrolyte maintained at 460 C., with the region of the electrolyte on the cathode side of a SO-mesh nickel screen barrier, interposed between the anode and cathode, .provided with an oxygen bubbling inlet, a fiow rate of the inlet oxygen of the order of 1350 cc./minute can produce a five-watt power density with about 10 percent utilization of the oxygen in the agitated cathode region.

It has been found that the addition of about 0.2 gram of manganese oxide to a 500 gram volume of the electrolytic medium enabled even a far less flow rate of inlet oxygen (namely, of the order of 1100 cc./minute) to provide about a 70 percent increase in air utilization (up to about 17 percent) and simultaneously with about a 40 percent increase in power density (up to about 7 watts). Similar results were attainable with substantially the same improvement in air-utilization increase and simultaneous power-density increase with the same reduced 1100 cc./ minute air inlet flow rate for about a one-gram iron oxide additive.

A corollary of the above is that for the same oxygen flow, the use of the additives of the present invention will enable operation of the cell at a lower temperature, if desired. It has been found that less than about one percent of additive is in all cases most adequate for the attainment of the results of the invention, and the specific values above given are well under this value.

As a further example, in another similar anhydrous alkaline hydroxide cell having 67 cm? of anode area and about five times this area in cathode area, being operated at about 475 C., each of cobalt, manganese and iron oxide additives in the proportions given above has been determined to enable about 14 watts power density to be obtained at an oxygen inlet flow rate of 2400 cc./minute and with at least about 16 percent air utilization. This is to be contrasted with the approximately 8 to percent of air utilization and 6 to 8 watts of power density that would be attainable with such a cell in the absence of the additive.

Further modifications will occur to those skilled in this art and all such are considered to fall Within the spirit and scope of the invention as set forth in the following claims.

What is claimed is:

1. A method of operating a fuel cell containing an alkali-metal hydroxide electrolytic medium, that comprises maintaining the medium at a temperature in excess of about 350 degrees C. and sufficient to render the medium molten and substantially anhydrous and in contact with an oxygen-containing atmosphere in an amount sufficient to provide in the medium at least one of alkalimetal peroxide and superoxide, said peroxide and superoxide functioning as substantially the sole fuel cell electrochemical oxidant, applying hydrogen-containing fuel to the fuel cell, introducing to the fuel cell an oxidizable metal selected from the group of transition metals capable of catalyzing at least one of said peroxide and superoxide formation and utilization in the medium, and drawing current from said fuel cell while holding the said medium at such temperature and while utilizing said oxidant electrochemically to provide substantially the entire current output of the fuel cell.

2. A method as claimed in claim 1, in which said group of transition metals consist of manganese and iron.

3. A method as claimed in claim 2, in which the volume of said oxidizable metals is not greater than about 1% of the volume of said medium.

4. A method as claimed in claim 2, in which the said temperature is adjusted in excess of about 400 degrees C.

5. A method as claimed in claim 1, in which said medium is disposed between an anode and a cathode in a housing and the region of said medium near the cathode is agitated to further the electrochemical reaction thereat.

6. A method as claimed in claim 5, in which the transfer of agitation from the cathode region to the anode region of said medium is impeded while permitting the passage of ions of said medium between said regions.

References Cited UNITED STATES PATENTS 3,026,364 3/1962 Jackson et al. 13686 3,134,698 5/1964 Neipert et al. 13686 3,280,014 10/1966 Kordesch et al 136-86 3,350,226 10/1967 Lieb et al. 13686 3,393,099 7/1968 Giner et al. l3686 3,419,900 12/1968 Elmore et al. 13686 ALLEN B. CURTIS, Primary Examiner 

